There are generally two classes of three-dimensional sound manipulation. The first class of manipulation involves processing of conventional recordings to create a wider sense of spaciousness, with sounds apparently emanating from beyond the position of the loud speakers. Such systems include the "SRS system," described in U.S. Pat. Nos. 4,748,669, 4,841,592 and 4,886,774, assigned to Hughes Aircraft Company, the "spatializer" products marketed by Desper Products, such as described in U.S. Pat. No. 4,308,423 (to Joel Cohen) and U.S. Pat. No. 5,412,731 (assigned to Desper Products, Inc.), the QSound System, described in U.S. Pat. Nos. 5,026,051, 5,046,097, 5,052,685, 5,105,462, 5,208,860, 5,371,799 and 5,436,975 (assigned to QSound Ltd.), and a system described by Robert W. Carver in U.S. Pat. Nos. 4,603,429 and 4,309,570. The techniques used in these systems are commonly referred to as "stereo spreading."
The second class of sound manipulation involves the arbitrary placement of single or multiple mono sound sources. This allows a computer to dynamically change the apparent location of sound. This technique involves more exact modeling of the human ear system, and in general requires a digital signal processor to implement localization filters.
The techniques of the first class of three-dimensional sound manipulation generally use a form of feedforward signal processing. For example, in the system described by Carver, an anti-phase electrical cross-coupling signal is introduced to eliminate cross-coupling between right channel signals which reach the left ear and left channel signals which reach the right ear. The anti-phase signal is designed to match the ratio of the transfer function from the speaker to the near ear to the transfer function from the speaker to the far ear. This introduction of an anti-phase electrical cross-coupling signal typically involves a delay of about two hundred microseconds and a frequency response roll off above one kilohertz. By canceling the cross-coupling paths, a signal that comes from the right speaker appears to be farther to the right only because the sound at the left ear is reduced in amplitude. There are several drawbacks to this technique. One is that room acoustics prevent effective cancellation because the sound arriving at the opposite ear is composed not only of sounds traveling along the path from the speaker to the opposite ear, but also includes sound reflected from walls or other objects in the room. Another drawback is that the electrical frequency response of a system using cross-cancellation is not flat, and this causes a sound reproduction to sound hollow.
In the "spatializer" system, rather than using cross-cancellation, a difference signal is derived by subtracting the right input from the left input. This difference signal is fed forward through a signal processing block consisting of a delay and low pass filter. The output of this filter is added to one channel and subtracted from the other channel. For monaural signals, the difference signal component is zero; therefore no processing takes place. Since most of the bass and lead vocal signals are panned to the center, frequency response aberrations which may occur in the Carver system generally do not occur in the "spatializer" system. For signals that appear in the left or right channels only, however, the frequency response is affected. This system works by shifting the phase of low frequencies between the ears. Since the primary localization mechanism below 0.5 kilohertz is inter-aural phase, images outside of the speakers can be obtained by increasing this phase shift. A drawback of this system is that it is optimized by trial-and-error selection of filter components. While it is easy to derive the ear response for a particular set of filter components, it is more difficult to start with the specification of the desired ear response and to derive the required filter components.